The activity of a normal, healthy heart consists of the synchronized contraction of the atria and ventricles of the heart. Blood is received into the atria, which contract, forcing the blood into the ventricles. Subsequent contraction of the ventricles then causes the blood to be pumped through the body and eventually returned to the atria. The contractions of the chambers of the heart are caused by coordinated electrical stimulation of portions of the cardiac muscle. The heartbeat cycle begins with the generation of an electrical impulse by a bundle of fibers located in the sinoatrial node of the heart, near the upper portion of the right atrium at the entrance to the superior vena cava. This impulse spreads across the atria, stimulating the atrial muscles and causing the atrial contraction which forces blood into the ventricles. An atrial contraction is manifested as the so-called "P-wave" in an electrocardiographic signal. The electrical impulse conducted through the atrial muscle is then received at the partition wall immediately beside the valve between the right atrium and right ventricle, at the atrio-ventricular or A-V node. The A-V node introduces a slight delay in the transmission of the electrical impulse to the ventricles. This A-V delay is typically on the order of 100-milliseconds or so. After the A-V delay, the electrical impulse is conducted to the ventricles, causing the ventricular contraction which is manifested as the "QRS complex" of an electrocardiographic signal. Subsequent repolarization and relaxation of the ventricular muscles occurs at the end of the cardiac cycle, which is manifested as the "T-wave" portion of an electrocardiographic signal.
For patients in which the above-described conduction of electrical impulses through the cardiac muscle is somehow impaired, a pacemaker can provide an artificial electrical stimulus where no natural electrical impulse is present. Thus, for example, a ventricular pacemaker can function to cause ventricular contractions in patients in which the natural electrical cardiac impulse is, for some reason, not transmitted across the A-V node. It is essential, however, that any artificial stimulating pulses be delivered at appropriated times, so that proper synchronization of atrial and ventricular action is maintained. In addition, it is known that electrical impulses being delivered to the cardiac muscle during the repolarization phase at the end of the cardiac cycle can cause the onset of tachyarrhythmias; it is therefore essential that the pacemaker be prevented from delivering stimulating pulses during the T-wave.
In order to maintain A-V synchrony, and to prevent delivery of pacing pulses at undesirable times, pacemakers are preferably capable of detecting either atrial activity, ventricular activity, or both, as manifested by the P-wave and QRS complex, respectively, of the electrocardiographic signal.
A wide variety of cardiac pacemakers are known and commercially available. Pacemakers are generally characterized by which chambers of the heart they are capable of sensing, the chambers to which they deliver pacing stimuli, and their responses, if any, to sensed intrinsic electrical cardiac activity. Some pacemakers deliver pacing stimuli at fixed, regular intervals without regard to naturally occurring cardiac activity. More commonly, however, pacemakers sense electrical cardiac activity in one or both of the chambers of the heart, and inhibit or trigger delivery of pacing stimuli to the heart based on the occurrence and recognition of sensed intrinsic electrical events.
The North American Society of Pacing and Electrophysiology (NASPE) and the British Pacing and Electrophysiology Group (BPEG) have adopted a three-letter code which is used to describe the operative modalities of pacemakers. The first letter of the three letter code designates the chamber or chambers of the heart to which the pacemaker delivers pacing pulses; an "A" in the first position designates atrial pacing, a "V" designates ventricular pacing, and a "D" designates both atrial and ventricular pacing. Similarly, the second letter position designates the chambers of the heart from which the pacemaker senses electrical signals, and this second letter may be either an "A" (atrial sensing), a "V" (ventricular sensing), a "D" (atrial and ventricular sensing, or an "O" (no sensing). The third letter position designates the pacemaker's responses to sensed electrical signals; the pacemaker's response may either be to trigger the delivery of pacing pulses based upon sensed electrical cardiac signals (designated by a "T" in the third position), to inhibit the delivery of pacing pulses based upon sensed electrical cardiac signals (designated by an "I" in the third position), or both trigger and inhibit based upon sensed electrical cardiac signals (designated by a "D"). An "O" in the third position indicates that the pacemaker does not respond to sensed electrical signals.
Thus, for example, a "VVI" pacemaker delivers pacing stimuli to the ventricle of a patient's heart, senses electrical cardiac activity in the ventricle, and inhibits the delivery of pacing pulses when ventricular signals are sensed. A "DDD" pacemaker, on the other hand, delivers atrial pacing stimuli to both the atrium and ventricle of the patient's heart, senses electrical signals in both the atrium and ventricle, and both triggers and inhibits the delivery of pacing pulses based upon sensed electrical cardiac activity. The delivery of each pacing stimulus by a DDD pacemaker is synchronized with prior sensed or paced events. Other well-known types of pacemakers include AOO, VOO, AAI, VDD, and DVI.
Those types of pacemakers which are capable of delivering pacing pulses to both chambers of the heart (e.g., DDD and DVI pacemakers) are collectively referred to as "dual chamber" pacemakers. Presently, dual chamber pacing is most often accomplished with two electrodes, one being disposed in the patient's atrium and the other in the patient's ventricle. Two electrodes are also typically required if pacing is to be performed in one chamber and sensing in the other, as in the case of VAT pacing.
The two electrodes required for dual chamber or VAT pacing may either be disposed on two separate transvenous leads, or disposed at different points along the length of a unitary transvenous lead. Although in earlier dual chamber pacemakers, separate electrodes were required to perform pacing functions and sensing functions, thus requiring a total of four electrodes for a DDD pacemaker, it is commonly the case in more recent pacemakers that a single ventricular electrode is used for pacing, sensing, or both pacing and sensing in the ventricle, and a single atrial electrode is used for pacing, sensing, or both pacing and sensing in the atrium.
One reason that two electrodes have heretofore been deemed essential when sensing is to be performed in both chambers of the heart relates to the disparity of relative magnitudes of naturally occurring atrial electrical signals and ventricular electrical signals. As previously noted, the ventricular contraction constitutes the dominant portion of each cardiac cycle, and the electrical signals associated with ventricular contraction are accordingly of a greater magnitude than the electrical signals associated with atrial contraction. When sensing is performed from a ventricular electrode, a sensed ventricular signal will typically have a peak value on the order of 5 to 25-millivolts or so, whereas an atrial signal will typically be indiscernible or have a peak value of only 1 to 2-millivolts or so. Thus, atrial signals are typically not strong enough to be sensed by a single electrode situated in the ventricle. On the other hand, while the larger magnitude ventricular signals may be detectible by an electrode situated in the atrium, using a single electrode to receive both atrial and ventricular signals leads to difficulties in distinguishing between atrial and ventricular events, which is crucial to maintaining synchronous operation of a pacemaker. In practice, therefore, separate atrial and ventricular electrodes have been preferred.
Similarly, two electrodes have heretofore been deemed necessary when pacing is to be performed in both chambers of the heart, since from a ventricular electrode the pulse amplitude required to stimulate the atrium is unacceptably high.
Certain disadvantages to the use of two electrodes for pacing and/or sensing have been recognized. One disadvantage is that ensuring the correct placement of two leads can be difficult. Due to differences in the respective physical configurations of the right ventricle and right atrium, achieving a stable and effective placement of an electrode is more readily accomplished in the ventricle than in the atrium. A ventricular lead may be lodged near the apex of the right ventricle, and may even be attached to the ventricular wall by means of a tined or "screw-in" lead or the like. There is no corresponding physical structure in the atrium, however, which can provide similar support for an atrial electrode. Atrial lead stability is a problem, and even an atrial electrode, once properly placed, is susceptible to being moved or dislodged.
Another disadvantage of utilizing two electrodes for dual chamber or VAT pacing is the cost of two leads, or a two-electrode lead, as compared with a single, one-electrode lead. Moreover, two-lead pacing systems are relatively more complex than one-lead pacing systems, and are accordingly more prone to failure due to lead placement and mechanical failure.
In view of the foregoing, it is believed by the inventor that a method of accomplishing dual chamber sensing and pacing with a single electrode would be of great benefit to physicians and patients alike.
It is accordingly a feature of the present invention that a pacemaker is provided which utilizes a single electrode for sensing both atrial and ventricular electrical signals.
It is another feature of the present invention that a pacemaker is provided which utilizes a single electrode for delivering electrical pulses for stimulating the atrium of a patient's heart, and for stimulating the ventricle of a patient's heart.
It is a further feature of the present invention that means are provided in a pacemaker for reliably distinguishing between atrial electrical signals and ventricular electrical signals which are received on a single electrode.